Capacitive touch input devices are utilized in a variety of applications. For example, transparent capacitive touch input devices placed in front of displays may be utilized with computers or portable devices to enable user interaction with displayed objects. Opaque capacitive touch input devices are used for track pads and other applications not requiring programmable displayed images behind the touch surface.
Capacitive touch input devices may include a touch sensitive electrode layer that enables determination of a point of contact such as over a display, at the location where a user touches the touch sensitive layer over the display. A group of sensing electrodes enable determining the X and Y location of the point of contact. The electrodes may be coupled to capacitance sensing circuitry including analog-to-digital converters that measure values associated with the electrodes, such as the capacitance, current, charge, impedance or voltage associated with the electrodes.
Many capacitive touch input devices use at least two electrode layers to measure two coordinates, e.g. (X,Y), of a touch location. For example, parallel sensing electrodes aligned along the X-axis formed on a first layer and parallel sensing electrodes aligned along the Y-axis formed on a different or second layer such that the electrodes on the first layer are formed orthogonally with respect to the electrodes on the second layer and form an overlapping matrix of addressable points of the touch sensor. Such two layer capacitive touch input devices provide good touch performance for many applications, but at increased manufacturing costs.
For some applications, a touch input device with a single electrode layer may provide adequate touch performance at a lower cost. However, single electrode layer touch devices tend to be more susceptible to certain sources of background interference as compared to two-electrode-layer capacitive input devices.
Mechanical deflection of the touch device is a source of background interference. For example, touch induced pressure against the device may cause mechanical deflection between sensing electrodes and an underlying ground plane.
Stray capacitance between the sensing electrodes and the palm of the hand or other body parts of the user is another source of background interference. This background is particularly problematic if there is a thick dielectric layer between the electrodes and touch surface. Doubling the dielectric layer thickness will halve the desired finger touch signal, but will have little effect on backgrounds due to palms and other body parts (because air gap has strong effect on background capacitance than dielectric layer thickness). Hence, the ratio of undesired palm background signal, relative to the desired finger touch signal, increases as the thickness of the dielectric layer between electrodes and the touch surface increases. Such background signals from stray capacitances from the palm and other body parts can distort measured touch coordinate positions, particularly in single layer electrode designs.